Field Capacity and Soil Water

Field Capacity and Soil Water

• Field capacity refers to the water-holding capacity of soil.
• After rainfall or irrigation, some water drains along slopes and percolates down.
• Some water reaches the water table due to gravity becoming gravitational water.
• The water retained by the soil is its field capacity or water-holding capacity.
• Soil structure and temperature affect field capacity.
• The effective soil depth along with clay content determines the water holding capacity.
• Wheat is the benchmark plant for assessing available water holding capacity.
• Rankings are estimated by soil texture, structure, and stone content in the root zone.
• The water content of the soil after downward drainage of gravitational water determines the field capacity
• Field capacity is the upper limit of soil water storage for plant growth and the capillary capacity of a soil.
• At field capacity, the soil water potential ranges from -0.1 to -0.3 bars.
• Soil texture and organic matter control water-holding capacity.
• Soils with smaller particles (silt and clay) hold more water due to the larger surface area.
• Organic matter content influences water-holding capacity, increasing it as the percentage increases.

Soil Pores and Their Function

• Very Small Pores (10%): involved in holding soil particles, crusts, and contributing to compaction.
• Very Large Pores (10%): Enable rapid water infiltration and air inflow, and are inhabited by ants and worms (0.5-5mm).
• Large Pores (25%): Facilitate water and air exchange, soften the soil, and improve root growth (0.03-0.5mm).
• Small Pores (30%): serve as storage for plant-available water (<0.075mm).
• Medium Pores (25%): Aid in drainage and water/nutrient flow to roots (0.03-0.075mm).

Water Potential Components

• Water potential relies on free energy within a system.
• Chemical potential refers to free energy per mole for non-electrolytes.
• Water potential represents the chemical potential of water, denoted by the Greek letter Psi (ψ).
• Pure water has a water potential of zero.
• Solute particles reduce the free energy of water, leading to a decrease in water potential.
• Water potential is always less than zero for solutions, hence expressed in negative value.
• Water potential is determined by three internal factors:
  • Solute potential (ψs): also called osmotic potential
  • Pressure potential (ψp): also called turgor potential
  • Matric potential (ψw): potential pertaining to water molecules adhering to soil particles and cell wall.
• In plant systems, matric potential is typically disregarded.
• Water potential is then simplified to: Ψw = ψs + ψρ

Osmotic Pressure

• Osmotic pressure equals osmotic potential in magnitude but has the opposite sign.
• Osmotic pressure results from solutes in a solution, which lower the water potential.
• Osmotic pressure is a quantitative index of decreasing water potential in a solution.
• Osmotic and osmotic potential are equal in numerical value but opposite in sign.
• Osmotic pressure has a positive sign
• Osmotic potential has a negative sign (ψs).
• Example: OP = 20 atm., Ψw = - 20 atm

Turgor Pressure

• Turgor pressure arises from the presence of water molecules within a plant cell.
• The potential created by turgor pressure is called pressure potential (ψp).
• Water potential in a normal plant cell (partially turgid) is expressed as: Ψw = ψs + Ψp
  • Fully turgid cell: ψw = Zero (highest)
  • Flaccid or plasmolysed cell: ψω = ψε (lowest)

Water Relations

• Water is a major element in cells and facilitates life processes.
• Water releases hydrogen for plants and is a product of photosynthesis.
• Water facilitates biochemical reactions in living tissue.
• Water transports inorganic nutrients, photosynthesis products, bases, and hormones in aqueous solutions.
• Water regulates leaf temperature and enables nutrient availability to plant roots.
• Water is essential for life.

Diffusion Pressure Deficit (DPD)

• Diffusion pressure of a solution is lower than its pure solvent
• Diffusion pressure deficit (DPD) is the difference between the diffusion pressure of a solution and its solvent.
• Higher solution concentrations increase DPD, while dilution decreases it.
• DPD measures water absorption ability and is also called suction pressure (SP).
• DPD relates to osmotic pressure (OP), turgor pressure (TP), and wall pressure (WP) and is expressed as: DPD = OP – WP, where WP = TP
• DPD = OP – TP
• O.P = T.P = D.P.D = 0 is a fully turgid cell
• D.P.D = O.P in fully plasmolysed cells because T.P = 0
• Plant cell DPD is influenced by both osmotic pressure and turgor pressure
• Higher osmotic pressure accompanied by lower turgor pressure results in a greater DPD, causing water to enter the cell
• Water moves from cell B to cell A (i.e., towards lower or more negative water potential) with a form of (-8-(-3) = -5 bars.
• Water entry depends on Diffusion Pressure Deficit.